MODEL RELEASED. Patient using a peak flow meter, a device used to measure, in litres per minute, the rate of exhalation of air from the lungs. The peak flow is measured by taking in a deep breath and then breathing out with maximum effort through the mouthpiece of the meter. The reading provides one indication of lung function; it is useful in assessing the severity of bronchospasm, a narrowing of the airways in the lungs which tends to lower the peak flow. The device is commonly used by asthma patients to monitor their response to treatment with bronchodilator drugs.